Key takeaways
- Short fibres align with flow, so a glass-filled part shrinks less along the flow than across it.
- Warpage is the visible result of that difference, not a separate defect with a separate cause.
- Gate position, wall transitions and rib layout change orientation more than the glass content does.
- Specify flatness on a datum and measure it in the agreed condition, or the first article will argue with the drawing.
Why glass changes the rules
Adding short glass fibres to a thermoplastic raises stiffness, improves creep resistance and reduces the coefficient of thermal expansion. It also makes the material anisotropic: the properties depend on the direction you measure them in. A 30% glass-filled grade can be several times stiffer along the fibre than across it.
The same alignment that adds stiffness changes how the part shrinks. Fibres resist contraction in their own direction, so the melt shrinks less along the flow and more across it. A flat plate with one gate therefore wants to shrink by different amounts in two directions, and the only way it can do that is to bow.
How the fibres end up where they do
In injection moulding, fibres align with the local flow direction. Near the surface, where the melt shears against the cold tool, alignment is strong. In the core, flow is slower and the orientation is more random or more transverse. The result is a layered structure, and the balance between skin and core decides the final shrinkage.
The gate is the strongest lever. A single edge gate produces a fan of flow and strong alignment towards the far edge. A centre gate produces radial flow, with fibres running outwards and turning as they meet the wall. Sequential valve gates, hot drops and a long thin flow path each leave a different pattern, and a weld line is a place where two patterns meet and orientation is poor.
From differential shrinkage to a bowed part
Warpage appears when shrinkage is uneven through the thickness or across the part. Uneven wall sections cool at different rates, and the thicker section shrinks more. Ribs pull the surface they sit on. A boss with a heavy wall acts as a hot spot. Glass makes each of these effects directional, so a part that would have shrunk evenly now curls in a predictable direction.
The direction is predictable enough to design around. A long, flat cover gated at one end usually bows along its length, with the ends lifting. A box with four walls and one gate often pulls into a diamond. Knowing which way it moves lets you add a rib, move the gate or build a small allowance into the tool instead of fighting the first shot.
What the glass content changes
More glass means more stiffness, lower shrinkage overall and a larger gap between flow and cross-flow shrinkage. It also means more abrasion in the barrel and the tool, a narrower processing window and a greater chance of fibre breakage at the gate. The table sets out the trend, with illustrative values for a 30% glass-filled grade.
| Glass content | Stiffness | Shrinkage pattern | Practical effect |
|---|---|---|---|
| Unfilled | Low | Near isotropic but high | Even shrink, soft part, easy flow |
| 15% short glass | Moderate | Mild directionality | Better stiffness, small warp risk |
| 30% short glass | High | 0.3% along flow, 0.9% across, illustrative | Warp becomes a design variable |
| 50% short glass | Very high | Strong directionality | Warp control needs gate and rib work |
| Long glass | Highest | Less directional, still layered | Tougher, harder to mould, gate wear |
Shrinkage values are illustrative and vary with grade, wall, gate and process. Use the material supplier's data and a mould flow study for a real tool.
Design moves that reduce warpage
Keep the wall even and the ribs thinner than the wall. A rib at 50 to 60% of the nominal wall stiffens the part without creating a hot spot on the far side. If a rib must be thicker, core it out or add a shut-off so the mass does not collect. Bosses should be connected to a wall with a gusset, not left as isolated cylinders.
Place the gate where the flow is balanced. A part that fills from one end has a long orientation gradient; a part filled from the centre or from two symmetrical gates has a shorter, more balanced path. Avoid gating into a thin rib, because the fibre breakage and the packing loss at that point will show as a weak, warped corner.
- Balance the flow path so the last point to fill is central
- Keep ribs at 50 to 60% of the wall and radius the root
- Put the gate on a thick section, not on a cosmetic face
- Expect a weld line opposite the gate and design around it
- Use a mould flow study before the tool is cut, not after the trial
Specifying and measuring flatness
A flatness callout without a datum and a measurement condition is an argument waiting to happen. State the datum, the area to which flatness applies, the clamping condition and the time after moulding. A glass-filled part can continue to move for hours or days as it cools and absorbs a small amount of moisture.
Measure with the part supported as it will be in service. A cover that is screwed down at four points will be pulled flat by the screws, and measuring it free will show a bow that never appears in the assembly. Agree whether the check is free state or assembled, and put that sentence in the inspection plan.
What to send with the drawing
Send the grade and glass content, the wall sections, the gate position if it is fixed, the flatness requirement and the assembly condition. Add the cosmetic faces and any area where a weld line is not acceptable. If the part is still in design, send the concept and say which dimensions are fixed.
- Resin grade, glass content and colour
- Nominal wall and all rib and boss sections
- Gate position, or the freedom to choose it
- Flatness, datum and free or assembled condition
- Weld line and cosmetic restrictions
- Annual volume and tool life expectation
Frequently asked questions
Why does a glass-filled part warp more than an unfilled one?
The fibres align with flow and resist shrinkage in that direction, so the part shrinks less along the flow than across it. That difference, combined with uneven walls and cooling, makes the part bow. An unfilled grade shrinks more overall but more evenly.
Does more glass always mean less warpage?
No. More glass lowers overall shrinkage but increases the difference between flow and cross-flow shrinkage, and it narrows the process window. Warpage usually depends more on gate position, wall balance and cooling than on the glass percentage alone.
Can warpage be corrected in the tool?
Sometimes. A small, predictable bow can be compensated by machining the tool, and gate or cooling changes can reduce it. A large or inconsistent bow is usually a design or process problem, and correcting it in the tool can move the problem to another dimension.
When should a mould flow study be run?
Before the tool is cut, once the gate position, wall sections and material grade are known. A fill and warp study will show the likely bow and the weld line positions. It is cheaper to change a gate in the study than in a hardened tool.
Sources and further reading
- ISO 527-4:2023, Plastics, determination of tensile properties, fibre-reinforced composites
- ISO 294-1:2017, Plastics, injection moulding of test specimens of thermoplastic materials
- ISO 20457:2018, Plastics moulded parts, tolerances and acceptance conditions
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
A glass-filled part that will not stay flat?
Send the drawing, the grade and the gate position. The engineering desk will review the wall balance and the flow path before the tool is cut.
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